Replace sys.exit() in non-main functions by exceptions

Also move all exceptions into one module

fdroidserver/exception.py

@@ -0,0 +1,44 @@
+class FDroidException(Exception):
+
+    def __init__(self, value=None, detail=None):
+        self.value = value
+        self.detail = detail
+
+    def shortened_detail(self):
+        if len(self.detail) < 16000:
+            return self.detail
+        return '[...]\n' + self.detail[-16000:]
+
+    def get_wikitext(self):
+        ret = repr(self.value) + "\n"
+        if self.detail:
+            ret += "=detail=\n"
+            ret += "<pre>\n" + self.shortened_detail() + "</pre>\n"
+        return ret
+
+    def __str__(self):
+        ret = self.value
+        if self.detail:
+            ret += "\n==== detail begin ====\n%s\n==== detail end ====" % self.detail.strip()
+        return ret
+
+
+class MetaDataException(Exception):
+
+    def __init__(self, value):
+        self.value = value
+
+    def __str__(self):
+        return self.value
+
+
+class VCSException(FDroidException):
+    pass
+
+
+class BuildException(FDroidException):
+    pass
+
+
+class VerificationException(FDroidException):
+    pass